The present invention relates to a process for manufacturing a solar cell device, particularly one having a solar cell structure formed of a semiconductor material such as a compound semiconductor material. An example of such a solar cell is an AlGaAs/GaAs solar cell.
AlGaAs/GaAs solar cells have a higher conversion efficiency than any other solar cells which have been developed for practical use. However, the GaAs substrate is expensive, and heavy, having a specific gravity twice that of Si. Moreover, GaAs has a low mechanical strength since it has a tendency to cleave.
To alleviate the above problems, the following proposals have been made.
(a) First Proposal (for reducing weight and cost). Al.sub.x Ga.sub.1-x As and GaAs have substantially the same crystal lattice structure (lattice dimension), so that there is no mismatch (disorder in the crystal structure) at the interface. It is therefore possible to form a single crystal, or a good quality. Accordingly, by epitaxially growing, p-n junction of GaAs and AlGaAs on a GaAs substrate, a crystal of a good quality is obtained, and a solar cell having a high conversion efficiency can be obtained. /
As x of Al.sub.x Ga.sub.1-x As approaches 1, the rate of etching by an acid or an alkali becomes higher compared with GaAs.
Utilizing these facts, AlAs or Al.sub.x1 Ga.sub.1-x1 As, with x1 close to 1, is epitaxially grown on a GaAs substrate, and then a GaAs layer of a first conductivity type, a GaAs layer of a second conductivity type, and an Al.sub.x2 Ga.sub.1-x2 As layer of the second conductivity type are formed. Here, x1 is made larger than x2, so that the etching rate of the Al.sub.x1 Ga.sub.1-x1 As layer is substantially higher.
After that, the layer of AlAs or Al.sub.x1 Ga.sub.1-x1 As is removed by etching to separate the epitaxially grown layers from the substrate to form a thin film AlGaAs/GaAs solar cell. Thus, the GaAs substrate can be re-used. Moreover, the weight of the solar cell can be reduced.
This method has the disadvantage that the thin film of AlGaAs/GaAs has a low mechanical strength, and is often broken during the process of electrode formation and the process of assembly.
(b) Second Proposal (for reducing weight and cost, and for improving the mechanical strength).
In this method, a silicon substrate is used. A layer of a material, such as Ge, having a crystal lattice structure (lattice dimension) equivalent to that of GaAs is formed by metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or the like. Then, a GaAs layer of a first conductivity type, a GaAs layer of a second conductivity type and an AlGaAs layer of the second conductivity type are formed. In this way, the mechanical strength is improved.
A disadvantage of this method is that GaAs single crystal of a good quality cannot be obtained. Accordingly, it is difficult to obtain a stable solar cell having a high conversion efficiency.